Frequency-domain optical coherence tomography (FD-OCT) systems employ light generated by a source such as a laser to collect data with respect to a sample. This data can be used to form cross-sectional images of biological tissue and other samples. Typically, lasers with fixed cavity lengths longer than 50 cm are used in FD-OCT systems. These fixed long-cavity length laser are relatively easy to construct using conventional fiber or free-space optical components.
When tuned over a wide band of wavelengths, lasers with fixed cavity lengths traverse longitudinal modes spaced by an optical frequency interval equal to Δυ=c/2 L, where L is the length of the laser cavity. Therefore, the longer the laser cavity, the more modes are available to sustain laser oscillation within a given optical tuning bandwidth. For example, when tuning over the wavelength range of 1260-1360 nm (238-221 THz), the typical operating range of lasers employed in FD-OCT systems, a laser with a fixed cavity length of 50 cm (longitudinal mode spacing=0.33 GHz) traverses more than 56000 modes. During a rapid sweep over such a large number of modes, intensity fluctuations are smoothed by the finite response time of the optical amplifier.
Unfortunately, tunable long-cavity lasers also have several significant disadvantages as FD-OCT light sources. Because of the long photon residence time inside the laser cavity, these lasers suffer from a severe trade-off between scanning speed and coherence length. Typically, the sweep repetition rate of such lasers is limited to 10-20 KHz at imaging depths greater than a few millimeters. Tunable long-cavity lasers also tend to be bulky and difficult to mass produce at low cost.
In light of the foregoing, there is a need for other types of lasers and related signal processing methods that are suitable for use in FD-OCT systems that overcome these disadvantages.